In order to support the widespread use of fuel cells in many areas of transportation and military use, it is necessary to develop methods of processing liquid fuels to generate hydrogen for use by fuel cells. There have been many successful efforts in the development of fuel processors that can work off sulfur-free methane with demonstrated long lifetimes. However, the fuels with the highest energy density, such as diesel, gasoline, and jet fuels, consist of a large amount of heavy hydrocarbons, including aromatics, as well as upwards of 0.5 wt % sulfur. Catalysts for high temperature reforming of these fuels are very susceptible to coke formation from these higher hydrocarbon species, as well as from sulfur poisoning, and thus it is very difficult to develop a fuel processor that can operate on this fuel directly.
It is possible to convert the heavy hydrocarbons to lighter ones through the process of pre-reforming, whereby the fuel is contacted with steam over a catalyst at temperatures about 100 to 300° C. lower than typical reforming temperatures in order to produce an equilibrium mix of methane, hydrogen, carbon oxides, and water. This lower temperature for pre-reformer can reduce, but not completely remove, the formation of coke during the pre-reforming process. The output stream from pre-reforming can then be reformed at high temperatures without concerns for coke mitigation with only methane present over well known catalysts. However, pre-reforming does not remove any sulfur in the input fuel; this sulfur can affect both the pre-reformer and reformer catalysts, as well as other processing units within the fuel processor or fuel cell.
Sulfur is generally removed from fuel in one of two means. The simplest process is the use of a sulfur-adsorbent bed, usually based on zinc-oxide. This bed will capture not only H2S but also sulfur-containing hydrocarbons, but will have a limited capacity for sulfur uptake. The other typical option used for sulfur removal is hydrodesulphurization, where the fuel is contacted with hydrogen and steam at pressures between 5-20 atm and 350-500° C.; this will strip sulfur from large molecules while leaving these mostly intact, generating a sulfur-free fuel and H2S. The latter species will still need to be removed by an adsorption bed prior to fuel reforming. Neither of these options are ideal for a fuel processor; the zinc oxide bed would require frequent maintenance, and an hydrodesulphurization system would be both energy intensive and very difficult to design for a small processor.
There is a need for a solution to convert heavy hydrocarbon, sulfur-laden fuels into hydrogen for a system with sufficient longevity without frequent maintenance cycles. A system that combines the benefits of pre-reforming